1. Field of the Invention
The present invention relates to a WDM (Wavelength Division Multiplexing)-PON (Passive Optical Network) system, and more particularly, to a self-healing WDM-PON system for detecting malfunction and degradation problems of an upstream/downstream light source along with disconnection and degradation problems of a trunk line and a distribution fiber, and to automatically recover from such problems.
2. Description of the Related Art
Typically, a WDM-PON system provides subscribers with super high-speed broadband communication services using unique wavelengths assigned to the subscribers. Therefore, the WDM-PON system can guarantee communication confidentiality and easily accommodates an additional communication service requested by individual subscribers or increased communication capacity. Further, the WDP-PON system easily increase the number of subscribers by further including a unique wavelength assigned to a new subscriber. However, the aforementioned WDM-PON system must use an additional wavelength stabilizer for controlling a CO (Central Office) and individual subscriber ends to stabilize a light source of a specified lasing wavelength and a wavelength of the light source. This results in a high costs assessed to the subscriber. The WDM-PON system is not commercially available due to the high costs assessed to the subscriber. In order to implement a cost-effective WDM-PON system, there have recently been developed a spectrum-sliced broadband light source capable of easily managing wavelengths. This includes a mode-locked Fabry-Perot laser with incoherent light, and a reflective semiconductor optical amplifier (SOA) as WDM light sources.
Typically, the WDM-PON system adapts a double-star structure to minimize the length of an optical line. In more detail, a single feeder fiber connects the CO with a RN (Remote Node) located in a nearby area of subscribers, and an independent distribution fiber connects the RN to the individual subscribers. A multiplexed downstream signal is transferred to the RN via a trunk fiber and is demultiplexed by a multiplexer/demultiplexer contained in the RN. The demultiplexed signal is then transferred to individual subscriber units via the distribution fiber. Upstream signals created from the subscriber units are transferred to the RN, and are applied to the multiplexer/demultiplexer contained in the RN. The upstream signals are multiplexed by the multiplexer/demultiplexer, and are then transferred to the CO.
The WDM-PON system transfers large amounts of data at a high transfer rate via wavelengths assigned to individual subscribers. In such systems, unexpected malfunction and degradation incidents of an upstream or downstream light source occur or disconnection and degradation incidents of a trunk line and a distribution fiber occur. This may cause the WDM-PON system to lose large amounts of data even though such an incident is maintained for a short period of time. Therefore, there is a need for the WDM-PON system to quickly detect such an incident and recover from the incident.
However, if such an unexpected incident occurs, a direct communication circuit between the CO and the subscriber unit is broken, such that a communication mode between the CO and the subscriber is disabled. To solve this problem, the WDM-PON system may further use a low-speed communication circuit, resulting in an additional cost for managing/monitoring the low-speed communication circuit located between the CO and each subscriber unit. The CO and the subscriber communicate with each other via the low-speed communication circuit in order to determine whether an unexpected incident occurs, and may use a predetermined period of time to inform an administrator of such an incident. This results in an increased communication failure notification time between the CO and the subscriber unit. Therefore, there must be developed a self-healing WDM-PON system that quickly recognizes either malfunction and degradation problems of an upstream/downstream light source or disconnection and degradation problems of a trunk fiber and a distribution fiber in an implemented optical link configuration. Further, such a self-healing system should automatically recover from the detected malfunction, disconnection and degradation problems.
Typically, a WDM optical communication network system configures a plurality of optical communication nodes arranged at regular intervals in the form of a ring network in order to automatically recover from unexpected problems. Such unexpected problems include a disconnection or degradation of a transmission optical fiber. There has been initially proposed a four-strand self-healing ring optical network composed of a two-strand working fiber and a two-strand protection fiber in order to implement two-way communication. With the increasing development of a two-way communication technique using one-strand fiber, there has recently been proposed a ring optical network. Such a system is composed of one-strand working fiber and one-strand protection fiber to reduce the number of transmission fibers and implement such two-way communication.
FIG. 1a is a block diagram of a conventional self-healing ring optical network. The self-healing ring optical network uses a protection switch method for recovering from a communication failure of the transmission fiber using a loop-back scheme. Individual nodes of the ring optical network system are comprised of optical add-drop multiplexer/demultiplexers (OADMs) 10a˜40a and 10b˜40b and 2×2 switching units 110˜180, which is used for protection switching. In this case, the OADM demultiplexes a multiplexed optical signal transferred via inside and outside ring fibers, drops a signal having a wavelength assigned to each node, modulates the signal having the wavelength according to transmission data, and multiplexes the modulated signal having the same wavelength along with other demultiplexed signals. The outside ring fiber 4 transmits optical signals having wavelengths λ1, λ2, λ3, . . . , λN in a clockwise direction. The inside ring fiber 2 transmits optical signals having wavelengths λN+1, λN+2, λN+3, . . . , λ2N in a counterclockwise direction.
FIG. 1b is a block diagram illustrating a protection switching scheme for a transmission fiber link according to the loop-back principle. As shown in FIG. 1b, if a communication failure occurs in the transmission fiber link, the optical network system adapts an optical signal as a loopback signal using two 2×2 optical switching units located at both ends of an erroneous link. The optical system then transmits the optical signal serving as the loopback signal in an opposite direction in such a way that the protection switching can be performed. For example, if a communication failure occurs in an optical fiber link for connecting the OADM 10a with the OADM 20a as shown in FIG. 1b, the optical signals λ1, λ2, λ3, . . . , λN transferred from the OADM 10a to the OADM2a 20a return to the OADM 10b via the switching unit 120. The OADM 10b then transmits the received optical signals in a counter clockwise direction via the inside ring fiber 2. The optical signals λ1, λ2, λ3, . . . , λN transferred via the inside ring fiber 2 are transferred from the OADM 20b to the OADM 20a in such a way that signal switching is performed.
If the ring network system is normally operating, the 2×2 optical switching units 110˜180 are in a bar state, a signal received at an input terminal i1 is transferred to an output terminal o1, and a signal received at an input terminal i2 is transferred to an output terminal o2. However, if the ring network system is operating abnormally, the 2×2 optical switching units 110˜180 are in a cross state. Thus, the signal applied to the input terminal i1 is transferred to the output terminal o2, and the signal applied to the input terminal o2 is transferred to the output terminal o1.
If the optical switching unit 130 shown in FIG. 1b is in a cross state, a signal passing through an erroneous link and optical signals having wavelengths λN+1, λN+2, λN+3, . . . , λ2N transferred from the OADM 20b to the OADM 10b in a counter clockwise direction are adapted as loop-back signals. The signal is thereby transferred via the outside ring fiber 4 in a clockwise direction, and is transferred from the OADM 10a to the OADM 10b via the optical switching unit 120. The optical switching unit located at a node nonadjacent to the erroneous node remains in a bar state without any state conversion.
The self-healing WDM ring optical network system configures a plurality of nodes in the form of a ring. Although a transmission fiber may be disconnected, nodes can still communicate with each other using another fiber arranged in the opposite direction to the disconnected transmission fiber. Thus, the self-healing WDM ring optical network system quickly recovers from a communication failure caused by the disconnected fiber, and thereby maintains a communication state between the nodes.
However, since the nodes are interconnected in the form of a ring, a number of steps must be performed. These include receiving common signals multiplexed/demultiplexed by a multiplexer/demultiplexer. Further the received common signals being output without any change, drop and receive a signal having a wavelength corresponding to each node, and add the same wavelength signal modulated with transmission data to the common signals. Thus, the aforementioned self-healing WDM ring optical network system must use an OADM using a high-priced wavelength division multiplexer/demultiplexer. If large amounts of an optical power is lost in multiplexing/demultiplexing signals at individual nodes, the self-healing WDM ring optical network system must also use an optical amplifier to compensate for the lost power, resulting in an increased system cost. Therefore, the conventional self-healing WDM ring optical network system is not applicable to the WDM-PON system focused on economical efficiency.